“My friend the majority leader is going to be remembered as the worst leader in the Senate ever,” McConnell said. “It makes me sad.”

Reid said McConnell could not be trusted to keep his word to move executive branch nominations through the Senate.

“I refuse to unilaterally surrender my right to respond to this breach of faith,” Reid said of McConnell. “I wait, I wait, but I’m not going to wait another month, another few weeks, another year for Congress to take action on the things that we have been doing for 200 and almost 40 years.”

The verbal firefight followed Reid’s announcement that he would set in motion a process to change the Senate’s rules to make it easier to vote on Obama administration nominees that have been blocked by GOP filibusters.

Reid said he planned to use a party-line vote to change the Senate’s rules so that nominees can be confirmed by a simple majority, thereby doing an end run around a Republican blockade of nominees to key boards that oversee Wall Street and labor relations.

The method for changing the rules would require a ruling from the presiding officer, likely to be Vice President Biden, declaring filibusters on such Cabinet or agency nominations invalid, followed by a vote requiring 51 votes to uphold the ruling.

After two previous threats to change rules on party-line votes — a move that critics have dubbed the “nuclear option” — Reid declared that this time he would do so. He set a key test vote beginning Tuesday with the nomination of the head of the new Consumer Financial Protection Bureau (CFPB).

“We have the votes to move forward on this,” Reid said after a 90-minute meeting Thursday afternoon with the Democratic caucus.

Reid’s declaration punctuated an emotional day for Reid, 73, and McConnell, 71, who have developed a particularly contentious relationship after nearly seven years of serving as leaders of their parties in the Senate. They spend nearly every day just a few feet apart, across a blue-carpeted aisle separating the two desks from which they have led their respective caucuses.

“These are dark days in the history of the Senate,” McConnell said late Thursday during the day’s second bickering match between the two leaders.

Reid’s move would dramatically affect the ability of the minority party in the Senate to block nominees it does not like, and the prospect of such a move rankled other senior Republicans. They threatened to cut off bipartisan negotiations on unrelated legislative matters and vowed to expand the use of bare-majority rule in the Senate if and when they regain the majority.

They singled out the long-stalled project to store high-level nuclear waste outside Las Vegas, a proposal that Reid has blocked by using every procedural tactic available.

One last-gasp effort at civility was set for Monday evening, when an unusual bipartisan caucus of the entire Senate is scheduled in the Old Senate Chamber.

Not used for formal sessions since 1859, that room has helped defuse previous moments of partisan rancor — including impeachment proceedings for the trial of President Bill Clinton — with senators emerging saying they had invoked the compromising spirits of Clay, Calhoun and Webster.

The GOP’s blocking tactics in recent years have escalated, including brief filibusters of nominations for defense secretary and CIA director that usually are not subject to party politics. In those cases, McConnell bowed to pressure from junior Republicans who have never tasted the majority.

Under similar pressure from junior Democrats who never served in the minority, Reid has threatened twice before to ignore the long-standing precedent that changing the Senate’s rules requires 67 votes. That supermajority level is otherwise reserved for ratifying international treaties and removing an impeached president from office, a level meant to ensure that Senate rules would not change with the political winds the way they do in the authoritarian House.

Just last January, Reid told The Washington Post that he personally still believed in a “60-vote threshold” to end filibusters and that his threat to change the rules on a party-line vote was designed to extract concessions from McConnell.

On Thursday, Reid said he reversed course because Republicans had reached new levels of obstructing nominations, violating the spirit of a deal the two leaders hatched in January to avert a similar showdown.

“Republican obstructionism has affected nearly every one of President Obama’s nominees,” Reid said. “No president can safeguard America’s national and economic security to the best of his or her ability without their chosen team in place.”

At issue are nominees for secretary of labor, administrator of the Environmental Protection Agency, the heads of the CFPB and the Export-Import Bank, and several members of the National Labor Relations Board (NLRB).

The most contentious nominations in question are those for the CFPB and NLRB. Republicans had been filibustering any nominee to lead the CFPB because of their opposition to the way the Dodd-Frank law regulating Wall Street had established the agency, and had been delaying nominees to the labor board. Fearing the two agencies had been left inoperable, Obama used his appointment power in January 2012 to temporarily fill those spots, rejecting decades of previous precedent that dictated a president could do so only while the Senate was adjourned for a lengthy period.

Federal courts have since ruled that those appointments were invalid, casting in doubt more than 1,000 rulings that the NLRB has issued in the past 18 months. The Supreme Court will hear a case on the constitutionality of those appointments in its next term.

If the Senate confirms those temporary appointees to full terms, the board could presumably reissue those rulings and render moot the debate over their validity.

Republicans said Thursday they were not willing to confirm the nominees appointed by recess appointments and urged Obama to send up new names for those posts. They also accused Democrats of yielding to a labor-union campaign to change the confirmation rules in the hope that the NLRB would be cleared of uncertainty.

“They seem to be in the pockets of the unions,” Sen. Orrin G. Hatch (R-Utah) said.

Several senior Democrats — led by Sen. Carl Levin (Mich.) — oppose the proposed rule changes, warning their junior colleagues that it would lead to Republican abuse when that party holds the majority.